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Full text of "Handbook Of Chemical Engineering - I"

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250                               CHEMICAL ENGINEERING
ing on the degree of precision of screening required, any degree of closeness of work can be obtained. In practice, however, in those grading industries where extra high precision is demanded and obtained, as in the abrasive industries, in which flat shaking screens are much favored, the ratio of length to width of screen ranges from two to four, the higher ratios being on the narrower screens. While oblong screens are favored in industries requiring precise grading such lengths do not run to extremes for with a load not too great on the screen the slow elimination of grains of almost the size of the aperture by extra screen length does not yield any appreciable benefit. In precise grading the ratio of length of screen to width runs from two to five. From 25 to 50 per cent of the rate of feed which has been indicated as practicable for capacity screening should be employed in precise screening, experience being the guide in this factor as well as in the one of screen length.
Screen Tests.—In arriving at the number of screens of any type to be employed screen tests with hand screens should be employed. The assumption that the same number and size of screens, even when the apertures from one to another differ by a fixed ratio, should be the same for all sizes, should only be used for rough calculations. Where much information has been collected and arranged on the way any certain material groups itself into ranges of sizes following comminuting or other operations reducing it to fragments smaller than those subjected to such reducing operation, then such data may be followed as a guide in estimating the number and size of screens to be employed in any series screening operation.
For rocks and ores an approximate screen test may be written, assuming using the 1.414 ratio, by reckoning the rock or ore all crushed to pass any size of a set in which 1.414 is the multiplier and divisor and reckoning the percentage by weight which will remain on the screen of next smaller size of opening as 27, that remaining on the third as two thirds of this figure, etc., the percentage remaining on any screen being two thirds of that of next larger size and the base for the computation being 27 per cent. Following this direction if material were crushed to pass a screen with 1.050 in. opening, there would be 27 per cent remaining on the 0.742 in. size, 18 per cent on the 0.545 size, etc. If material were crushed to pass the 0.0041 in., 150-mesh size, there would be 27 per cent resting on the 0.0029 in., 200-mesh size and 73 per cent of —200-mesh size.
Revolving screens may conveniently be divided into three classes: (1) The ordinary slow-revolving designs much favored in rock, gravel and ore grading and crushing plants; (2) centrifugal reels with high speed of rotation and much used in flour milling and allied industries. Sulphur, graphite, ground drugs, pigments and other materials are also graded in centrifugal reels; (3) disc graders— in the parent or Rich machine the screen is built up of a series of annular rings or discs. The grader is bound together by a series of tie rods which pass through the inner lugs of the ring. (U. S. patent 892,825, July 7, 1908, shows a mechanical equivalent of the Eich device in a long coiled spring and by adjusting the pitch of the spring any desired amount of opening between the coils can be obtained.) The screening space is, of course, the space between the rings or the spring